Abstract: The 7^th member of Yanchang Formation (Chang 7) in Ordos Basin is rich in shale oil resources.However,a poor understanding of the differences of characteristics between shale and mudstone has hindered the exploitation of shale oil.Various methods including direct observation of core and thin section and analyses of XRD,Rock-Eval,TOC,chloroform bitumen "A" extraction as well as major and trace element analysis are used to study the sedimentological,mineralogical and geochemical characteristics of shale and mudstone in Chang 7 Member of Triassic Yanchang Formation in Ordos Basin,Northwest China.The results indicate that the shale shows darker color,obvious lamination and smooth surface,while the mudstone has grey color,no fissiles,and non-smooth cut surface.The organic matters in shale are mainly enriched along beddings,the organic matters in the mudstone are scattered.The clay content is high in the shale,while quartz content is relatively high in the mudstone.The average TOC of the shale (18.5%)is more than 5 times that of the mudstone and the hydrocarbon generation potential in shale is 5~8 times that in mudstone.More aromatic hydrocarbons and less saturated hydrocarbon is contained in the component of chloroform bitumen "A" of shale than that of mudstone.The shale was deposited in an environment with lightly lower paleosalinity and shallower paleo-water depth than that of the mudstone.The shales in Chang7 strike NW-SE and have thickness in the range of 15-30 m.The macroscopic distribution of shale oil is controlled by shale facies,therefore the comparative study between shale and mudstone is of great significance for accurate prediction of organic-rich shale and guiding shale oil/gas exploration.

Key words: shale, mudstone, tight oil, Yanchang Formation, Ordos Basin